Question:
A certain household has consumed 250 units of energy during a month. How much energy is this in joules?
Answer:
1 unit of energy = 1 kilowatt hour (kWh).
∵ 1 kWh = 3.6 × 106 J
∴ 250 units of energy = 250 × 3.6 × 106 = 9 × 108 J
Work and Energy
Q 1.
A body is thrown vertically upwards. Its velocity goes on decreasing. Write the change in kinetic energy when its velocity becomes zero.
Q 2.
A 60 kg person climbs stairs of total height 20 m in 2 min.Calculate the power delivered.
Q 3.
Name a machine that transforms muscular energy into useful mechanical work.
Q 4.
Flowing water can rotate a turbine.What type of energy is used up by the turbine ?
Q 5.
Two girls,each of weight 400 N,climb up a rope through a height of 8 m.We name one of the girls A and the other B.Girl A takes 20 s while B takes 50 s to accomplish this task.What is the power expended by each girl ?
Q 6.
Define potential energy.Derive an expression for the potential energy of a body of mass m,at a height h above the surface of the Earth.
Q 7.
How does the wind move the blades of a wind-mill ?
Q 8.
An object of mass 15 kg is moving with a uniform velocity of 4 m/s.What is the kinetic energy possessed by the object ?
Q 9.
What is the form of energy possessed by a running car?
Q 10.
When is work done by a force zero ?
Q 11.
Name two forms of mechanical energy.
Q 12.
What do the machines need for their working ?
Q 13.
Name the type of energy possessed by a raised hammer?
Q 14.
A force of 7 N acts on an object.The displacement is,say 8 m,in the direction of the force,[in Fig.].Let us take it that the force acts on the object throughout the displacement What is the work done in this case ?
Q 15.
A freely falling object eventually stops on reaching the ground.What happens to its kinetic energy ?
Q 16.
Why do some engines require fuels like petrol and diesel ?
Q 17.
Name the device which converts electrical energy into mechanical energy.
Q 18.
Does the transfer of energy take place when you push a huge rock with all your might and fail to move it? Where is the energy you spend going?
Q 19.
A boy of mass 55 kg runs up a flight of 40 stairs,each measuring 0.15 m.Calculate the work done by the boy
Q 20.
What is the work done by a constant force ?When is it positive,negative or zero ?
Q 21.
Fill in the blanks :
Ability to do work is called _______.
Q 22.
The mass of a ball A is double the mass of another ball B.The ball A moves at half the speed of the ball B.Calculate the ratio of the kinetic energy of A to the kinetic energy of B.
Q 23.
Can there be displacement of an object in the absence of any force acting on it ? Think.Discuss this question with your friends and teacher.
Q 24.
A boy of mass 50 kg runs up a staircase of 45 steps in 9 s.If the height of each step is 15 cm,find his power.Take g =
.
Q 25.
Write an expression for the work done when a force is acting on an object in the direction of its displacement.
Q 26.
The velocity of a body moving in a straight line is increased by applying a constant force F,for some distance in the direction of the motion. Prove that the increase in the kinetic energy of the body is equal to the work done by the force on the body.
Q 27.
Is a person doing any work by holding a suitcase ?
Q 28.
What type of energy is possessed by a cricket ball just before being caught by a fielder ?
Q 29.
A coolie carries a load of 50 kg on his head and walks on a level road upto 100 m.What is the work done by him ?
Q 30.
Can a body have energy without having momentum ?
Q 31.
A porter lifts a luggage of 15 kg from the ground and puts it on his head 1.5 m above the ground.Calculate the work done by him on the luggage.
Q 32.
Fill in the blanks :
A force acting on a body shows a displacement in the direction of the force. It is said _______ is done on the body.
Q 33.
How much work is done on a body of mass 1kg whirling on a circular path of radius 5m?
Q 34.
Define average power.
Q 35.
A mass of 10 kg is at a point A on a table. It is moved to a point B. If the line joining A and B is horizontal, what is the work done on the object by the gravitational force? Explain your answer.
Q 36.
A force of 7 N acts on an object. The displacement is, say 8 m, in the direction of the force. Let us take it that the force acts on the object through the displacement. What is the work done in this case?
Q 37.
The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?
Q 38.
Two friends Ram and Shyam, each having weight of 40 kg, go for rock climbing. Ram climbs to a height of 3 m in 10 s. and Shyam covers the same height in 12 s. Is the work done by Ram and Shyam equal ? or not ? Compare the power of Ram and Shyam.
Q 39.
Is work a scalar or vector quantity?
Q 40.
A pendulum of length 1m and with a bob of mass 1 kg swings in a vertical plane. Find the work done (from the below said choices) by weight of the bob when it moves from position A to B (as shown in figure). (Take g = 10 m/sec^2)
a. 4J
b. -4J
c. -5 J
d. 5 J
Q 41.
If the momentum of a body is increased
n times, its kinetic energy increases (a) n times (b) 2n times (c) √n times (d) n
2 times
Q 42.
A motor pump does same amount of water by lifting underground water in shorter time than a hand pump. What do you infer from this statement?
Q 43.
A body of mass 15 kg undergoes downward displacement of 40m under the effect of gravitational force. How much work is done? (take g = 10 m/s
2)
Q 44.
A body of mass 120 g is taken vertically upwards to reach the height of 5m. Calculate work done. (Take g = 10 m/s
2)
Q 45.
What is the kinetic energy of an object ?
Q 47.
whether or not work is done in the light of your understanding of the term ‘work’.
(a) Suma is swimming in a pond.
(b) A donkey is carrying a load on its back.
(c) A wind-mill is lifting water from a well.
(d) A green plant is carrying out photosynthesis.
(e) An engine is pulling a train.
(f) Food grains are getting dried in the Sun.
(g) A sailboat is moving due to wind energy.
Q 48.
A battery lights a bulb.Describe the energy changes involved in the process
Q 49.
An electric heater is rated 1500 W.How much energy does it use in 10 hours ?
Q 50.
When is work done by a force positive ?
